The invention relates to a fuel injection pump of the type including a chamfer-edge-controlled fuel pump for internal combustion engines having an axially and rotationally movable pump piston guided in a pump cylinder, which pump piston includes in its outer surface two diametrically opposite control bores and recesses that terminate in chamfered control edges which recesses are in continual communication with the pump operating chamber. Further, these recesses can be connected with a low pressure chamber through two control bores lying opposite each other in the wall of the pump cylinder to terminate the effective delivery stroke, and of these recesses the first can be opened shortly before the second by its associated control bore and thus serve as an element of a connecting line from the pump operating chamber to the low pressure chamber which is provided with a throttle point.
One such fuel injection pump is already known (DT-OS 1,576,466, FIG. 5), in which the necessary pressure drop in the pump operating chamber and in the injection line is delayed by a throttled preliminary shut-off shortly before the actual shut-off to terminate the injection, in order to avoid cavitation in the injection system and late injections by the fuel injection valves. The pump piston of this pump has two recesses which terminate in chamfered control edges, of which the first recess, which can be opened by its associated control bore shortly before the second, is itself formed as a throttle point and is machined into the outer surface of the piston as a flat control groove. Achieving a throttling that is the same in every load position is practically impossible with this type of groove, especially when there are several pump pistons in multiple cylinder pumps, because an absolutely equal depth and breadth of the groove along the entire control edge cannot be produced economically. In small pumps with their associated extremely flat grooves, there is an additional difficulty of production, namely the problem of the influence of piston play, because such play changes the effective cross-section of the groove and thus also changes the throttling effect of the flat groove. Also problematic is the flow-rate dependence of the throttle effect at the control bore, and furthermore it must be seen as a disadvantage that the flat groove simultaneously determines the beginning point for the preliminary shut-off (by means of its position) and the throttling itself (by means of its cross section).
In another exemplary embodiment of the described fuel injection pump (DT-OS 1,576,466, FIG. 1), both recesses are provided with first and second control edge, between which is located the recess which causes the throttled shut-off control. The machining of this type of recess presents the same difficulties as that of the previously mentioned flat grooves discussed earlier.
Another construction which deviates from known fuel injection pumps has a longitudinal bore formed as a throttle bore, which serves as the only connection between the pump operating chamber and the recesses that are provided with control edges. This longitudinal bore serves to damp the pressure waves inside the pump piston that are caused by the shut-off. With this throttle, which is to be construed as a throttle for the entire quantity of fuel shut-off, however, the necessary throttled preliminary shut-off cannot be obtained, because the bore cannot be made small enough.
In another exemplary embodiment of this pump the desired preliminary shut-off is obtained by the throttle bore in the wall of the pump cylinder adjacent to the two opposite control edges. Because of the notch effect of the small throttle bore, this construction leads to cracks in the cylinder wall and thus to failure of the pump.